I am a final year PhD in UK in a scientific discipline and I am writing my thesis. I have already published 2 journal papers and I have submitted 2 more papers for publications. My department does not allow a "publications-based" thesis so I need to write a thesis in the context of a book etc.
As I write my thesis in a couple of places, especially in the literature review, I tend to reuse sentences from my publications. I try to restate them a bit, maybe change some words for their synonyms etc. but it is practically me writing about what I wrote before. It goes without saying that I cite/quote me at the end of a passage if I say something non-obvious (I found that slightly funny. :) ). The problem is that occasionally I am just explaining for instance how a certain estimation technique works; in that case I cite the original authors and not myself. The syntax in those occasions though is practically the same as the original passage I used in my publication; as I have explained it once and was consider good, I find no reason to reinvent myself (I do a mild rewording as I mentioned but that is quite insignificant). Same things goes for listings. I do cite my paper in the beginning of a big list as the list's source but the list itself is almost identical as the one in "my" paper; in those cases I don't use quotations, just attribution "".
Is there an obvious guideline? The basic definition of plagiarism "reproducing the work of another person's as your own" is not (directly) applicable to me because I am the other person (almost *); if I am using other people's work I do cite them but I don't cite myself, citing them, in quotation marks!
I am a bit "fuzzy" about how not to plagiarize myself in my thesis (I have had no problem regarding my journal publications).
(* In all publications mentioned I am the first -but not sole- author.)